The improvement of steering stability and ride quality is required for a vehicle.
In the blending of a rubber composition for a sidewall, blending is carried out so that a complex modulus E* is increased (for example, E* is 3 MPa to 6 MPa) in order to improve steering stability and blending is carried out so that the complex modulus E* is decreased (for example, E* is 2 MPa to 3 MPa) in order to improve ride quality. That is, when, in the blending of a rubber composition for a sidewall, blending is carried out so that a complex modulus E* is increased in order to improve steering stability, it results simultaneously in the deterioration of ride quality; therefore, the steering stability and ride quality has been in the relation of trade-off.
Japanese Unexamined Patent Publication No. 2004-106796 discloses a rubber composition for a sidewall comprising a polybutadiene rubber having a large amount of linear component and blended, a rubber composition for a clinch comprising a polybutadiene rubber having a large amount of linear component and/or a syndiotactic 1,2-polybutadiene, and a pneumatic tire for an automobile having a sidewall and/or clinch made of the rubber composition for a sidewall and/or the rubber composition for a clinch, in order to reduce rolling resistance and improve steering stability (motion performance) of a vehicle, secure requisite rigidity because of high complex modulus even if rubber thickness is reduced and carry out light weight and low fuel cost without decreasing the motion performance. However, since its rubber sheet is thick, orientation is inadequate and there is room for improvement in decreasing its effect by half.
Japanese Unexamined Patent Publication No. 2006-63143 discloses a rubber composition for a sidewall or a clinch comprising rubber components comprising a polybutadiene rubber comprising syndiotactic 1,2-polybutadiene crystals and a tin-modified polybutadiene rubber in order to improve cut resistance and crack growth resistance and the low fuel cost of a vehicle, and a sidewall or clinch made of the rubber composition for a sidewall or a clinch. However, there is room for improvement in improving both steering stability and ride quality.
Japanese Unexamined Patent Publication No. 2006-281744 discloses a pneumatic tire comprising a base tread prepared by laminating rubber sheets obtained by extruding a rubber composition comprising a diene rubber comprising a syndiotactic 1,2-polybutadiene at a thickness of at most 1 mm, in which a complex modulus E*a in a tire circumferential direction measured at a temperature of 70° C. and a dynamic distortion of 2% is 3 to 6 MPa and tan δ is 0.05 to 0.1, in order to improve ride quality and steering stability without increasing rolling resistance. However, Japanese Unexamined Patent Publication No. 2006-281744 relates to a base tread and its use is different from the present invention and the blending of a rubber composition is also different.
Although many patent literatures disclose a pneumatic tire having a sidewall made of a rubber composition for a sidewall comprising a syndiotactic 1,2-polybutadiene, it is a pneumatic tire comprising a sidewall integrally extruded. When an extruding gauge is thick, staple fibers are hardly oriented in an extruding direction, the effect of anisotropy is little and it is difficult to improve steering stability and ride quality in good balance.